High speed fluid jets are widely used for special cutting and drilling applications. Studies have shown that the jet cutting effect is greatest when a fluid jet of relatively high density, such as water, is exhausted into a low density medium, such as air. Studies on the cutting effect of liquid jets in air have shown that the cutting effect is maximized at particular standoff distances between the nozzle exit and the workpiece. With conventional arrangements, one such distance is between 200-300 times the nozzle orifice diameter (the so-called "far zone"); another enhancement of cutting effect is observed at 10-20 nozzle orifice diameters (the so-called "near zone"). (See N. C. Franz, "The Influence of Standoff Distance on Cutting with High Velocity Fluid Jets," Paper B-3, Second International Symposium on Jet Cutting Technology, Apr. 2-4, 1974, St. John's College, Cambridge, England.)
It has been observed that when the jet/medium density ratio is small, such as a ratio of 1:1 for a water jet exhausted into a water medium, the cutting effect decreases rapidly with distance from the jet nozzle exit. This inherent limitation on the cutting effect of submerged fluid jets has hindered their use in certain important applications, such as in petroleum drilling, where the jet would be submerged in a high-density medium, such as drilling mud.